Question about license implications of embedding Tectonic (esp. engine_xdvipdfmx)

[hahnlee]

Hello, first of all, thank you for this amazing project — I deeply respect the amount of work and care that's gone into it.
That said, I have a question that I’m having trouble figuring out:

I understand that Tectonic itself is under the MIT license. However, as stated in the LICENSE file:

Elements of the system from which it is derived are licensed under an extremely wide variety of open-source licenses.

In particular, I noticed that the engine_xdvipdfmx crate appears to include source code from (x)dvipdfmx. Given this, I’m wondering:

If I embed Tectonic (e.g., as a library in my own application), would this cause any GPL-style copyleft implications — especially due to the licensing of the original (x)dvipdfmx code?

I want to make sure I'm not inadvertently triggering any license incompatibilities or viral licensing effects by using Tectonic in this way.

[CraftSpider]

I am not a lawyer, so take this with a grain of salt. TL;DR - the situation here is murky, but not much different from using any other LaTeX application as far as I can tell. I can't say anything definitive on whether it's okay or not.

In detail:
The answer to this is complicated. Conventionally, using GPL source as part of a project makes your project GPL. However, this depends on what exactly is considered an individually copyrightable item - using a GPL tool doesn't make everything you do GPL, for example. Other TeX engines such as TeX-Live appear to take the same stance that the tectonic repository has so far, which is that xdvipdfmx is effectively an independently compiled tool we happen to use, so the rest of the project isn't GPL, only the bit that's 'part of' the copy and usage of xdvipdfmx. You can see this in how they (and us) distinctly keep the GPL disclaimer and license for files in that directory, but other directories have different licenses, despite possibly using xdvipdfmx indirectly through their respective engines. For tectonic, pdf_io and engine_xdvipdfmx are GPL, but other things only interact through the API of the engine, so are not using the code themselves. I don't know how this would hold up legally, however.

The best option here would honestly to be able to switch to the pdf crate at some point, then rewrite the dvi portions from-scratch without referring to the original source, only its input/output or public documentation of the format. Then we could remove our dependence on GPL code, and the associated complexities. This is, however, a long way off, if ever.

[hahnlee]

Thank you for your detailed and kind reply :)